Electroluminescent (EL) devices are structures which emit light when subject to an applied electric field. The usual model for the physical process in a semiconductor used in this way is through the radiative combination of electron-hole pairs which are injected into the semiconductor from opposite electrodes. Common examples are light-emitting diodes based on GaP and similar III-V semiconductors. Though these devices are efficient and widely used, they are limited in size, and are not easily or economically used in large area displays. Alternative materials which can be prepared over large areas are known, and among the inorganic semiconductors most effort has been directed to ZnS. This system has considerable practical drawbacks, primarily poor reliability. The mechanism in ZnS is believed to be one where acceleration of one type of carrier through the semiconductor under a strong electric field causes local excitation of the semiconductor which relaxes through radiative emission.
Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminescence. The practical difficulty with these materials is, as with ZnS, their poor reliability, together with difficulties in deposition of the organic layers and the current-injecting electrode layers. Techniques such as sublimation of the organic material suffer from the disadvantage that the resultant layer is soft, prone to recrystallisation, and unable to support high temperature deposition of top-contact layers. Techniques such as Langmuir-Blodgett film deposition of suitably-modified aromatics suffer from poor film quality, dilution of the active material, and high cost of fabrication.
An electroluminescent device utilising anthracene is disclosed in U.S. Pat. No. 3,621,321. This device suffers from high power consumption and low luminescence. In an attempt to provide an improved device, U.S. Pat. No. 4,672,265 describes an EL device having a double layer structure as its luminescent layer. However, the suggested materials for the double layer structure are organic materials which suffer from the disadvantages mentioned above.